USMLE – Cataracts
Cataracts are opacities in the crystalline lens. They are a major cause of visual loss and blindness throughout the world. Among the many causes of cataracts are metabolic diseases such as diabetes mellitus, physical agents (ultraviolet light trauma, radiation therapy), ocular diseases (uveitis, glaucoma, intraocular tumors, retinitis pigmentosa), viruses (rubella), skin diseases (atopic dermatitis, scleroderma), drugs (corticosteroids), and aging. The most common type is idiopathic and develops in older individuals so-called senile cataracts.
Cataracts are examined by ophthalmologists using a slit lamp biomicroscope. This permits observation of many of the same changes in vivo that the pathologist sees under the microscope after the lens is removed.
The most common change in the aging lens is nuclear sclerosis or compression of the lens fibers in the central (nuclear) portion of the lens. The borders of the individual nuclear fibers are indistinct, and the nucleus as a whole stands out from the cortex more distinctly than normal. As the process progresses, the nucleus may become brown (brunescent cataract). The earliest changes in the cortex are the appearance of wedgelike or spokelike opacities that occur first at the lens equator then progress into the anterior and posterior cortex (cortical cataract). Vacuoles often appear in the cortex of the lens, and clefts between lens fibers show an accumulation of opaque debris of degenerated lens material (Morgagni globules) .
Another common type of age-related cataract is marked by the development of granular opacities in the zone immediately anterior to the posterior capsule. These spread toward the periphery (posterior subcapsular cataract). A lens with an early or immature cataract frequently has an increased osmotic pressure as a result of the degenerated lens material and may imbibe water and swell (intumescent cataract). Eventually the entire lens may become involved in the degenerative process (mature cataract). The sclerotic nucleus may sink in the cortex when the latter becomes entirely liquefied (morgagnian cataract). Occasionally, cataractous lenses may shrink after the lens capsule and epithelium degenerate, and lenticular debris passes into the aqueous humor (hypermature cataract). As the lens capsule becomes more permeable, allowing lens material to escape, the lenticular debris is engulfed by macrophages that may obstruct the aqueous outflow, resulting in phacolytic glaucoma. Cataractous lenses may eventually calcify; less commonly, lens material is partially or completely reabsorbed, leaving the residual lens capsule. If cataract surgery is performed and some lens epithelium is left behind within the capsule, these cells may produce abnormal, opaque lens fibers that a appear as large globules.
On histologic section, abnormalities of the epithelium may be seen. The most common finding is migration of lens epithelium under the posterior lens capsule. The epithelial cells may enlarge and appear vacuolated (bladder cells), they may undergo necrosis and be partially or totally absent, or they may undergo fibrous metaplasia. After routine extracapsular extraction of a cataract by a surgeon, the remaining anterior and posterior capsules of the lens may become apposed, while the epithelium at the periphery forms abortive lens fibers having a doughnut shape (Soemmering ring cataract).